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Abstract

The muon longitudinal polarization ${P}_{L}$ in ${K}_{L}\ensuremath{\rightarrow}\ensuremath{\mu}\overline{\ensuremath{\mu}}$ decay is investigated in the standard model. The induced $\overline{s}\mathrm{dH}$ vertex responsible for ${P}_{L}$ turns out to be unexpectedly "large." The measurement of ${P}_{L}$ above \ensuremath{\sim} ${10}^{\ensuremath{-}3}$ will, therefore, be a signal of the existence of a rather light Higgs boson (${M}_{H}\ensuremath{\lesssim}10$ GeV). It is also shown that the mechanism of guage-dependence cancellation between Higgs-boson-exchange and box diagrams works even in the standard model.